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Yield stability refers to how stable the yield of an agricultural system is over time from one year to another. An agricultural system with high yield stability will output about the same amount of food each year. An agricultural system with low yield stability will output very different amounts of food each year. There are many factors affecting yield stability. The yield stability of an agricultural system has important human consequences.
Resource Depletion
Crops and animals need a variety of resources to grow and thrive. If these resources become depleted and scarce, then yield can decline.
Soil nutrients are one very important resource. For example, grains such as maize (corn) take nitrogen from the soil. If maize is grown on the same plot of land each year, then the nitrogen in the soil will gradually be depleted, preventing the maize from growing successfully. Nitrogen fertilizer can be applied to the soil to help the maize succeed. The use of nitrogen fertilizers in agriculture worldwide is so extensive that it has affected the entire global nitrogen cycle.
Fresh water is another important resource. Both plants and animals require water. In many places, agriculture has been supported by "fossil water". In contrast with rain water or water from snow melting, fossil water does not replenish itself each year. Once fossil water has been depleted, other sources of water must be found, or the agricultural system cannot be sustained in its original form.
Weather Changes
If the weather (mainly temperature and precipitation patterns) is approximately the same each year, then, all else equal, approximately the same crops can be grown each year. However, many regions experience quite different weather conditions from one year to the next. Additionally, regions worldwide are experiencing gradual shifts in weather conditions due to global climate change.
Biodiversity
Biologically diverse ecosystems tend to have more stable yields. This is because when some event happens that could reduce yields, the event typically only affects some of the species within the system. For example, if rainfall is unusually low one year, but some of the crops planted are well-suited to low rainfall, then those plants will have high yields. If rainfall is high the next year, then those plants will have low yields, but other plants will have high yields, making up for it. So no matter how much rain falls each year, there will be some crops with high yields, and thus a fairly stable total yield. In contrast, if the system has less diversity, and all of the crops are well-suited for one specific amount of rainfall, then the yield will be either very high (if that specific amount of rain falls) or very low (if some other amount of rain falls). Thus less diverse systems have less stability.
A striking historical example of the importance of biodiversity to yield stability is the Irish potato famine. In this example, low biodiversity lead to a catastrophically low yield.
Consider This...
The Irish Potato Famine
In the mid-1800’s, many people in Ireland were trying to support themselves and their families on very small pieces of land – often just a few acres. For them, the only crop that could provide enough calories was the potato. And so, they planted lots of potatoes, and not much else. This was a very difficult livelihood, but it was largely feasible.
Then, in 1845, potato blight made its way to Ireland from the United States. The blight was devastating to the potato crop, destroying as much as a third or a half of it. The consequences for the Irish people were catastrophic. The people hit the hardest were those whose agriculture was focused mainly on growing potatoes – the people whose agriculture had very little biodiversity. To make matters worse, these people already faced very difficult living conditions. With the devastation of their potato crops, they had little else available.
The potato blight caused a great famine, affecting a large portion of the Irish people. By the end of it, the island’s population fell from 8 million to around 6 million. About one million people died, and another million people emigrated to other places. Cities on the east coast of the United States, such as Boston, New York, Philadelphia, and Baltimore, all gained large Irish populations which they retain to this day.
It is important to understand that the causes and consequences of the famine were both ecological and social. The famine would not have occurred if the blight did not affect potatoes so severely, or if Irish agriculture had more biodiversity. The lack of biodiversity there was largely due to the social factors which caused the people to have such small plots of land to farm. Meanwhile, the blight was introduced to Ireland through a social process: shipping across the Atlantic Ocean. (Originally, the potato itself was introduced via trans-Atlantic shipping as well: the potato is native to South America.) Finally, the consequences of the famine would have been different if there was different support for the people affected by the blight. There was some support, which saved lives; this support could have been greater or lesser. Thus, the case of the Irish potato famine illustrates both the importance of biodiversity to yield stability and the idea that agriculture is a coupled human-environment system.
Let's learn more about biological diversity in agriculture and the value this brings.